The Biosynthesis of Retinal Cell Membranes is being explored with new techniques of immunochemical subcellular fractionation. This approach uses antibodies against opsin and a minor large molecular weight protein of the rod outer segment (ROS). Antibodies are isolated by new techniques of affinity chromotography and are being used as specific reagents to immunoprecipitate membranes bearing newly synthesized rhodopsin and newly synthesized large protein. These antibodies will be used to test two hypothetical pathways for the transport of membrane proteins: 1) Opsin and the large protein of ROS are transported concurrently to the base of the outer segment on membraneous "schleppersomes" composed almost entirely of rhodopsin. This pathway implies that the stoichiometry and molecular interactions of ROS membranes are established at the site of synthesis. 2) Opsin and the large protein of ROS are transported on complex membrane surfaces arising from vesicles or cisternae of the endoplasmic reticulum and are shuttled to the base of the ROS where assembly selectively excises the specific ROS membrane proteins and recycles or catabolises the ER membranes. If the former hypothesis is correct, then isolated membranes bearing newly synthesized opsin should be composed almost entirely of opsin comparable to the structure of the simple membranes of ROS. If the second hypothesis is correct, then newly synthesized opsin will be transported on complex membrane surfaces. The isolation of these intermediates during transport of opsin should generate insights about membrane protein transport in other cell systems as well. Histologic studies of the retinal dystrophy induced by the retinotoxic fern, bracken (Pteridium aquilinum) are continuing. To date, in a few animals studied we have been able to confirm evidence of a specific retinopathy induced by the fern. Further studies of this dystrophy will be continued. BIBLIOGRAPHIC REFERENCES: Papermaster, D.S., Converse, C.A. and Zorn, M., Immunochemical characterization of a new large protein in isolated rod outer segment membranes. Exp. Eye Res. (1976). In press. Papermaster, D.S., Converse, C.A., and Siu, J. Membrane biosynthesis in frog retina: Opsin transport in the photoreceptor cell. Biochemistry 14: 1343 (1975).